The pre-freezing process of the lyophilizer is to solidify the free water in the solution to give the product after drying the same form as before drying to prevent irreversible changes such as foaming, concentration and solute movement during vacuum drying, and minimize the material caused by temperature Reduced solubility and changes in life characteristics.
There are two pre-freezing methods for the solution: the pre-freezing method in the lyophilization box and the pre-freezing method outside the box.
The pre-freezing method in the box is to directly place the product on the multi-layer shelf in the lyophilizer, and freeze by the freezer of the lyophilizer. When a large number of vials and ampoules are lyophilized, it is convenient to enter and exit the box. Generally, the vials or ampoules are placed in several metal trays, and then packed into boxes, in order to improve heat transfer. Some metal trays are made into a removable bottom type, and the bottom is removed when entering the box, so that the vial is directly in contact with the metal plate of the freeze-drying box; for non-drawable bottom trays, the bottom of the tray is required to be flat to obtain product uniformity. The large plasma bottles using the spin-free method should be frozen in advance and then put into the box for freezing after adding a metal rack for heat conduction.
There are two methods for pre-freezing outside the box: some small freeze dryers do not have a device for pre-freezing products, and can only use low-temperature refrigerators or alcohol and dry ice for pre-freezing. The other is a special spin-freezer, which can freeze large bottles of products into a shell-like structure while rotating, and then enter the freeze-drying box.
The pre-freezing process of the lyophilizer:
When the temperature of the aqueous solution drops to a certain level, according to the eutectic concentration of the solution, ice begins to freeze in the weakly concentrated solution. This temperature is called the freezing point. Generally speaking, the freezing point is controlled by the concentration and decreases with the concentration. When the temperature of the solution is lower than the freezing point, a part of the solution will crystallize out, and the concentration of the remaining solution will rise, so the freezing point drops, and then continue to cool, the ice crystals increase with cooling, and the concentration of the remaining solution Increase with it. However, when the temperature drops to a certain point, all the remaining solution freezes. At this time, the frozen material is mixed with ice crystals, and the temperature at this time is the eutectic point.
After the solution needs to be supercooled to the freezing point, after crystal nuclei are generated in it, the free water will begin to crystallize in the form of ice, and at the same time it will release the heat of crystallization to make its temperature rise to the freezing point. As the crystal grows, the concentration of the solution increases. When the eutectic concentration is reached and the temperature drops below the eutectic point, the solution will all freeze.
In addition to the nature of the solution itself, the number and size of crystal grains in the solution crystal are related to the rate of crystal nucleation and crystal growth. The two factors, the rate of crystal nucleation and the rate of crystal growth, change with temperature and pressure. Therefore, we can control the number and size of crystal grains in solution crystallization by controlling temperature and pressure. Generally speaking, the faster the cooling rate, the lower the supercooling temperature, the more the number of crystal nuclei formed, and the crystal will be frozen before it can grow. At this time, the more the number of crystal grains formed, the finer the crystal grains; conversely, the crystal grains The smaller the number, the larger the crystal grains.
The shape of the crystal is also related to the freezing temperature. When it starts to freeze around 0°C, ice crystals are hexagonally symmetrical and grow forward in the directions of the six major axes. At the same time, several secondary axes will appear. All ice crystals are connected to form a network structure in the solution. As the degree of supercooling increases, ice crystals will gradually lose the hexagonal symmetrical form of capacity recognition. In addition, the number of nucleation is large and the freezing speed is fast, which may form an irregular dendritic shape. They have any number of axial cylinders. Unlike the hexagonal crystal form, there are only six.
The crystalline unit formed by freezing of biological fluids (such as blood plasma, muscle slurry, vitreous humor, etc.) is often similar to the type of ice crystals formed by a single-component aqueous solution. The type of crystallization mainly depends on the cooling rate and the concentration of body fluids. For example, when plasma, muscle slurry, etc. freeze under normal concentration, hexagonal crystalline units are formed at higher sub-zero temperatures and slow cooling rates, and irregular dendrites are formed when rapidly cooled to low temperatures. Crystal.
Cell suspension (such as red blood cells, white blood cells, sperm, bacteria, etc. suspended in distilled water, plasma or other suspension media). When freezing slowly at high sub-zero temperatures, a large amount of ice grows in the suspension, squeezing the cells between two icicles In the narrow pipeline between, the suspension medium in the pipeline is concentrated by the precipitation of water and the solute is concentrated, and the water in the cell penetrates the cell through the cell membrane, which in turn causes the concentration of the solute in the cell. At the same time, the growth of extracellular ice will also force cellular material to shrink and deform. But at this time, the cells do not freeze. When it freezes quickly at low temperatures, intracellular ice will form inside the cell. The size, shape and distribution of ice are related to the cooling rate, the presence or absence of the protective agent, the nature of the protective agent and the content of water in the cell. Generally speaking, the faster the cooling rate and the lower the temperature, the more ice is formed in the cell . The addition of a non-permeable protective agent to the suspension can reduce the number of ice formed in the cells during rapid freezing.
The form of solution crystallization has a direct effect on the rate of lyophilization. The void left by the ice crystal sublimation is the escape channel of water vapor during the subsequent ice crystal sublimation. The large and continuous hexagonal crystal has a large void channel after sublimation, and the resistance of water vapor escape is small, so the product dries fast, and vice versa. And the discontinuous spherical ice crystal channel is small or discontinuous, and the water vapor can escape only by diffusion or permeation, so the drying speed is slow. Therefore, only considering the drying rate, slow freezing is better.
In addition, the freezing rate is also related to the type, capacity and heat transfer medium of the freezing equipment.
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